Various types of retractable landing gear have been devised for model aircraft, particularly those operated by radio control. Some types are activated by an electric motor driven servo coupled to the landing gear by linkage. Others have an individual electric motor on each landing gear unit, with a reduction gear drive to retract and extend the leg. The primary problem with the electrically driven system is that, in the event of any type of electrical failure the landing gear is locked in place. If the gear is retracted in flight the aricraft must make a wheels up landing, with resultant damage to the structure.
Some systems use fluid pressure, such as air or Freon, to operate the landing gear. One such system is disclosed in U.S. Pat. No. 3,739,519, in which each landing gear unit is activated by an individual double acting cylinder mounted on the outside of the unit. Fluid pressure is supplied through a four way valve to retract and extend the landing gear selectively, the valve being activated by a servo. In present radio control equipment servos have become very reliable and the particular servo controlling the landing gear is used very little, so it is not subject to heavy wear. However, in the fluid pressure system there are numerous hoses and connections and a pressure leak is more likely to be a cause of failure. With the system in the above mentioned U.S. Patent, in which the landing gear is driven in both directions by pressure, any leak can result in the landing gear being stuck in its current position.